The present invention relates to a charge pumping circuit, and more particularly, it relates to a charge pumping circuit suitably used in a feedback system such as a PLL (phase locked loop) or a DLL (delay locked loop).
In general, a charge pumping circuit is used for generating a signal for controlling a voltage control oscillator (VCO) or a voltage control delay circuit (VCD) in a feedback system such as a PLL or a DLL. FIG. 6 shows the configuration of a conventional charge pumping circuit. When a signal UP is activated, a switch 101 is turned on and a current is supplied from a current source 102, namely, what is called a push operation is performed. On the other hand, when a signal DN is activated, a switch 103 is turned on and a current is drawn by a current source 104, namely, what is called a pull operation is performed. Such a current concerned with the push/pull operation is subjected to filtering processing by a low-pass filter 105 so as to generate a voltage Vo. The voltage Vo is used as a control signal for the VCO or the VCD (see, for example, Japanese Laid-Open Patent Publication No. 2000-82954 (p. 6, FIG. 6)).
In the control of each switch in the charge pumping circuit, switching noise is caused in the current by the push/pull operation. This switching noise will now be described with reference to FIG. 7.
In, for example, a switch constructed from a pMOS transistor, before turning on the switch, a charge of output capacitance is discharged through the fringe capacitance or the like of the switch and moves to a gate side, namely, what is called feedthrough noise is caused. Furthermore, after turning on the switch, the charge of the output capacitance is discharged through the gate capacitance or the like of the switch and moves to the gate side, namely, what is called injection noise is caused (corresponding to a period A of FIG. 7). Therefore, an output current is not supplied to a load capacitor immediately as a specified current Io but is used for charging the fringe capacitance and the gate capacitance for a while, and a remaining charge is charged in the load capacitor (corresponding to a period B of FIG. 7). The output voltage value of a current source is changed with time due to the influence of the on resistance of the switch if the current source is not an ideal current source, and hence, a state where the specified current Io cannot be attained lasts (corresponding to a period C of FIG. 7).
Next, in the instant of turning off the switch, the charge stored in the fringe capacitance and the gate capacitance is output at a stroke (which corresponds to feedthrough noise and injection noise), and the output current exceeds the specified current Io for a short period of time (corresponding to a period D of FIG. 7). Thereafter, the feedthrough noise and the injection noise are converged and the value of the output current is converged to zero (corresponding to a period E of FIG. 7).
When the charge pumping circuit is operated at high speed, a charge error derived from the switching noise is considerably large as compared with the charge moving through the push/pull operation. For example, it is assumed that the current value of the current source is 5 μA and the charge pumping circuit is operated at 250 MHz. In this case, a charge supplied in one switching operation is 5 f coulomb (=5 μA×1 ns). On the other hand, assuming that the fringe capacitance of the switch is 1 fF and the switch is operated at 4 V, a charge derived from feedthrough noise is 4 f coulomb (=4 V×1 fF). In this case, the charge error derived from the noise is substantially the same as the charge supplied from the current source. In other words, the charge moving through the push/pull operation of the charge pumping circuit includes an error derived from noise.
The significant point is that a charge error derived from noise is different depending upon the polarity of a transistor. In particular, between an n-channel transistor and a p-channel transistor, the relationship between a control voltage for turning-on and turning-off and the threshold voltage of the transistor is different, and hence, the quantity of a charge derived from charge injection noise is largely different. Owing to the characteristics of transistors, it is actually impossible to suppress the occurrence of the feedthrough noise and the injection noise. Furthermore, since the injection noise is changed in accordance with a power supply and process variation, it is extremely difficult to equalize the switching noise between a p-channel transistor and an n-channel transistor. Accordingly, since switching noise is asymmetric depending upon the polarity of transistors in the conventional charge pumping circuit, it is actually impossible to secure balance or relationship of a given ratio between a charge charged through a push operation and a charge discharged through a pull operation.